A field effect transistor (FET) is a three-terminal device having a gate electrode, source electrode, and a drain electrode.
The field effect transistor is an electronic active device in which voltage is applied to a gate electrode, a current flowing through a channel layer is controlled to switch the current between a source electrode and a drain electrode.
In particular, an FET which uses as a channel layer a thin film formed on an insulating substrate formed of ceramic, glass, plastic, or the like is called a thin film FET (thin film transistor, TFT).
The above-mentioned TFT has an advantage of being easily formed on a substrate having a relatively large area because the TFT is manufactured using thin film technology, and is widely used as a driving device in a flat panel display device such as a liquid crystal display device.
More specifically, in an active liquid crystal display device (ALCD), TFTs formed on a glass substrate are used to turn on/off individual image pixels.
Also, in a prospective high performance organic LED display (OLED), it is expected that current drive of pixels using TFTs is effective.
Further, a higher performance liquid crystal display device has been materialized in which a TFT circuit having a function of driving and controlling the whole image is formed on a substrate adjacent to an image display region.
The TFT most generally used now is a Metal-Insulator-Semiconductor Field Transistor (MIS-FET) device. This device uses a polycrystalline silicon film or an amorphous silicon film as the material of a channel layer.
These days, oxide materials receive attention as materials applicable to a TFT channel layer.
For example, development of TFTs which use as a channel layer a polycrystalline thin film of a transparent conductive oxide using ZnO as the main component is actively made.
The above-mentioned thin film can be formed at a relatively low temperature, and thus, the thin film can be formed on a substrate such as a plastic plate and a film.
However, due to scattering at the interface of polycrystalline particles, the electron mobility can not be made high.
Further, because the shape and interconnection of the polycrystalline particles greatly differ depending on the method of forming the film, the characteristics of the TFT device vary.
Further, K. Nomura et al., Nature 432, 488 (2004) reports a thin film transistor using an In—Ga—Zn—O-based amorphous oxide.
This transistor can be formed on a plastic or glass substrate at room temperature.
Further, normally-off transistor characteristics with a field effect mobility of approximately 6-9 have been obtained.
Further, this transistor is characterized by being transparent to visible light.
As a gate insulating layer of a thin film transistor, SiO2, SiNx, or the like is generally used.
With regard to a transistor which uses an oxide for a channel layer as well, use of those gate insulating layers is under consideration.
On the other hand, as a conventional transistor having a gate insulating film containing Ga as the main component, Japanese Patent Application Laid-open No. 2005-268507 discloses an FET using GaN as a channel layer.
However, this prior art uses a nitrogen compound excellent in crystallinity for the channel layer.
Japanese Patent Application Laid-open No. 2003-086808 discloses that in a TFT using crystalline ZnO as a channel layer, LiGaO2 or (Ga1-zAlz)O2 is used as an insulating layer.
However, the insulating layer described in Japanese Patent Application Laid-open No. 2003-086808 is a crystalline thin film, and, from the viewpoint of lattice matching between the channel layer and the insulating layer, materials for the respective layers are selected.
By the way, with regard to a thin film transistor using an In—Ga—Zn—O-based amorphous oxide, attempts are being made to materialize a thin film transistor having a large ON current by using a gate insulating layer having a high permittivity such as HfO2 or Y2O3.